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><H1
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><A
NAME="AEN94"
>3.1. Various parts of an operating system</A
></H1
><P
>A UNIX operating system consists
	of a <I
CLASS="GLOSSTERM"
>kernel</I
> and some
	<I
CLASS="GLOSSTERM"
>system programs</I
>.  There are also some
	<I
CLASS="GLOSSTERM"
>application programs</I
> for doing work.
	The kernel is the heart of the operating system.
	
		<A
NAME="AEN100"
HREF="#FTN.AEN100"
>[1]</A
>
		
	It keeps track of files on the disk, starts programs and runs them
	concurrently, assigns memory and other resources to various
	processes, receives packets from and sends packets to the network,
	and so on.  The kernel does very little by itself, but it provides
	tools with which all services can be built.  It also prevents anyone
	from accessing the hardware directly, forcing everyone to use the
	tools it provides.
		<A
NAME="AEN102"
HREF="#FTN.AEN102"
>[2]</A
>
	This way the kernel provides some protection for users from each
	other.  The tools provided by the kernel are used via
	<I
CLASS="GLOSSTERM"
>system calls<I
CLASS="GLOSSTERM"
>.  See manual page section 2 for more 
	information on these.  </I
></I
></P
><P
>The system programs use the tools provided by the kernel to
	implement the various services required from an operating system.
	System programs, and all other programs, run `on top of the
	kernel', in what is called the <I
CLASS="GLOSSTERM"
>user mode</I
>.
	The difference between system and application programs is
	one of intent: applications are intended for getting useful
	things done (or for playing, if it happens to be a game),
	whereas system programs are needed to get the system working.
	A word processor is an application; <B
CLASS="COMMAND"
>mount</B
>
	is a system program.  The difference is often somewhat blurry,
	however, and is important only to compulsive categorisers.</P
><P
>An operating system can also contain compilers and their
	corresponding libraries (GCC and the C library in particular under
	Linux), although not all programming languages need be part of
	the operating system.  Documentation, and sometimes even games,
	can also be part of it.  Traditionally, the operating system has
	been defined by the contents of the installation tape or disks;
	with Linux it is not as clear since it is spread all over the
	FTP sites of the world.</P
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><A
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>[1]</A
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><P
>In fact, it is often mistakenly considered
		to be the operating system itself, but it is not.
		An operating system provides many more services than a
		plain kernel.</P
></TD
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><A
NAME="FTN.AEN102"
HREF="x94.html#AEN102"
>[2]</A
></TD
><TD
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><P
>I always think of this as a form of encapsulation
		which may help those of you with an object oriented programming
		background to visualise it better.</P
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